With portable devices becoming increasingly small-sized, lightweight, and high performance, lithium secondary batteries, which are used as a main power source or a backup power source therein, have been required to have high capacity.
Among portable devices, in digital still cameras, which have been in great demand recently, the main power source is removed from the main body quite often for a long period of time when not in use unlike mobile phones, in which the main power source is less likely to be removed from the device. Additionally, devices like digital still cameras have a longer operating life. Therefore, for a backup power source of digital still cameras, both high capacity and excellent charge and discharge cycle performance are required. Also, its size has to be small. Thus, for the backup power source, coin-type lithium secondary batteries with a diameter of 1 cm or less, and coin-type lithium primary batteries with a diameter of 2 cm or less are often used. Such a battery includes an electrode formed of a molded body in pellet form.
For the negative electrode active material of the lithium battery, Si (silicon)(4199 mAh/g)-type materials, which achieve high capacity, have been examined. Lithium batteries using SiO as the negative electrode active material have been put in practical use as a backup power source for mobile phones and digital still cameras.
However, non-carbon-type negative electrode materials such as the silicon-type materials undergo significant volume change when lithium ions are absorbed and desorbed. For example, in the case of silicon simple substance, silicon theoretically expands 4.1 times the original size at its maximum lithium absorption. On the other hand, in the case of graphite, using its intercalation reaction, lithium is intercalated between the layers of graphite. Therefore, the expansion rate of graphite is about 1.1.
Thus, when silicon-type materials are used as the negative electrode active material, with the significant volume change of the active material, gaps are created between the active material particles, decreasing the negative electrode portion that effectively contributes to battery capacity. The volume change also causes cracks to the active material particles, micronizing the active material particles. The micronization of the active material particles creates space between the particles, disconnecting the electron conductive network based on the contact between the particles. Therefore, the negative electrode portion not contributing to the electrochemical reaction increases, the internal resistance increases, and the charge and discharge capacity declines. This may cause insufficient battery performance.
To solve such problems, for example, Japanese Laid-Open Patent Publication No. 2004-178922 (document 1) has proposed mixing particles containing a compound including silicon atoms with vapor deposited carbon fiber, and covering at least a portion of the surface of the particles containing the compound including silicon atoms with a carbonaceous material.
Japanese Laid-Open Patent Publication No. 2005-222933 (document 2) has proposed a negative electrode material containing a carbon-type negative electrode active material with a specific surface area of 1 m2/g or more, a binder of styrene butadiene rubber, and carbon fiber with a fiber diameter of 1 to 1000 nm. Document 2 also discloses that the resistivity of the negative electrode at 25° C. is preferably 0.5 Ωcm or less.
In documents 1 and 2, the electrode is made by mixing the electrode material with water or an organic solvent to obtain a paste, and applying the obtained paste to the current collector. In such an electrode, a thin material mixture layer containing an active material is carried on the current collector, and the current collector is attached to the material mixture layer by a binder. When the material mixture layer is thin, contacts between the active material particles can be kept easily in the case of a material mixture layer containing a conductive agent such as vapor deposited carbon fiber, and a material mixture layer containing a combination of carbon fiber and styrene butadiene rubber, compared with a conventional material mixture layer using carbon black such as acetylene black as the conductive agent. Further, in the case of the particles containing a compound including silicon atoms with at least the portion of the surface thereof covered with a carbonaceous material, charge and discharge cycle performance, and low temperature performance can be improved to a certain extent. Also, by decreasing the resistivity of the negative electrode, charge and discharge cycle performance can be improved to a certain degree.
On the other hand, in the case of coin-type batteries, a thick molded body in pellet form made by compression-molding a material mixture (granulated material) in a mold is used as the electrode. Such an electrode undergoes significant degree of expansion and contraction. Therefore, in the electrode made of the molded body, it is hard to keep the conductivity in the molded body, compared with the electrode made of a current collector and a thin active material layer formed thereon.
To be specific, when the active material expansion and contraction are repeated by charge and discharge, compared with the electrode including the current collector and the thin material mixture layer carried thereon, the electrode made of the molded body undergoes a high degree of expansion. Thus, even at least a portion of the active material particle surface is covered with a carbonaceous material, contact between the active material particles, i.e., the conductivity between the particles, cannot be kept just by mixing the active material particles with the vapor deposited carbon fiber, and charge and discharge cycle performance declines significantly.
Further, the molded body is usually molded with a high density to a certain degree, usually to secure the battery capacity and strength. Generally, the resistivity of the molded body is low with a high density, and high with a low density. With high density, the resistivity easily varies depending upon the mixing ratio of the materials included in the molded body. With low density, the resistivity easily varies depending upon the conditions of the molded body.
In the case of the active material with less volume change during charge and discharge (for example, graphite), with less density change of the molded body while charge and discharge, charge and discharge cycle performance can be improved by decreasing the resistivity of the molded body at the time of molding. That is, when making comparison between a molded body including an active material with less volume change and a conventional conductive agent such as acetylene black, and a molded body including an active material with less volume change and carbon fiber as the conductive agent, the latter molded body achieves a low resistivity, and cycle performance improves.
On the other hand, with the active material that undergoes significant expansion during charge such as Si simple substance, the density change of the molded body during charge and discharge is significant as well. Even with the active material expansion during charge, by discharge, the active material contracts to the size at the time of pre-charge. However, the molded body expanded due to the active material expansion during charge does not contract to the pre-charge state even discharged. Thus, in the molded body after discharge, the density at the time of molding is not kept, and gaps increase (that is, the density decrease) compared with the pre-charge state, and the contacts between the active material particles, i.e., the conductivity between the active material particles, cannot be kept. Therefore, decreasing the resistivity of the negative electrode molded body before battery assembly can only achieve a certain degree of effects. That is, even the resistivity of the molded body is decreased at the time of molding, unless the resistivity of the molded body during discharge cannot be decreased, charge and discharge cycle performance cannot be improved.